Masters of any Arcanum can translate an improvised spell into a Rote.
Creating a Rote requires Mastery of the Arcanum in question — the mage needs to know the Arcanum’s most advanced principles before being able to codify a spell into a Rote.
Masters often include personal flourishes in their Rotes, and so is it sometimes possible to tell which variant of a spell a mage uses based on how she performs a Rote.
In order to create a Rote, the mage needs to have cast the spell in question many times and be familiar with the Imago she will use to make the Rote. She then performs it, slowly and deliberately, infusing every gesture and action with magic, until the Rote is solidified and the Imago fixed and perfect in her mind.
The mage can create a Rote out of any spell she can cast, provided that she has Mastered all of the Arcana to be included. She needs to choose a Skill to encode and fix the Imago in her mind. Doing so is an Extended Action (Arcanum + the encoded Skill, each roll equals three hours of study and practice, 5 successes required per dot of Arcana involved in the spell).
The mage can devote three hours at a time to this working over a period of weeks — the successes need not be accrued sequentially. The player spends one Mana for each roll. The usual rules for failure apply.
If the player fails one of these rolls, the character must either break off the attempt or take a Condition, which cannot resolve until the Rote is complete or the mage gives up.
Once the character has mastered the Imago, the player spends a point of Willpower and an Arcane Experience, and the Rote is complete.
The character can then teach it to other mages, or use the Prime 1 spell Scribe Grimoire to write it down and allow others to cast it.